We have been using several serial and USB TC-08 with grounding.
We have recently procured two new USB TC-08 loggers that do not have the grounding option, and we get reading fluctuations of over 30C.
We cannot stop testing for the time it would take to ship this unit to the UK and back.

Please indicate what could be done to minimize these fluctuations and insure we would get the correct temperature readings from the K-Type Thermocouples.

We can arrange a returns authorisation for you to use when you are ready.

In the meantime, here are some ways to isolate ground noise.
Isolate the thermocouples from any metal surface.
If using a laptop, try grounding the chassis of the laptop to the metal surface you are measuring from.

Switch mode PSU noise can be alleviated by runing the laptop on battery or earting the chassis as mention before.

michael wrote:In the meantime, here are some ways to isolate ground noise.Isolate the thermocouples from any metal surface.

Some thermocouples are installed directly onto metal, and we need to measure the metal's temperature, in yesterday evening test they were not connected and no fluctuations occurred.

When the fluctuations occur most channels show a drop of ~30C and one channel shows similar gain.
Would it be safe to presume that the data that we usually log (~80% of the time) is the correct temperature and the shorter period data is erroneous data? Can we rely on the data already accumulated through very expensive and time consuming tests?

The outer metal part of the USB cable is the ground. You can pick up a ground point here. The important thing is to prevent a potential earth difference between the laptop and the metal surface you are measuring.

Noisy laptop PSU can cause a floating ground problem but is easily remedied by either earthing or running the laptop on battery.